1. Field of the Invention
The invention relates generally to the field of wellbore instruments and well logging methods. More specifically, the invention relates to systems and methods for synchronizing various instrument clocks in an instrument string disposed in a well.
2. Background Art
Well logging instruments are devices configured to move through a wellbore drilled through subsurface rock formations. The devices include one or more tools and other devices that measure various properties of the subsurface rock formations and/or perform certain mechanical acts on the formations, such as drilling or percussively obtaining samples of the rock formations, and withdrawing samples of connate fluid from the rock formations. Measurements of the properties of the rock formations may be recorded with respect to the instrument axial position (depth) within the wellbore as the instrument is moved along the wellbore. Such recording is referred to as a “well log.”
Well logging instruments can be conveyed along the wellbore by extending and withdrawing an armored electrical cable (“wireline”), wherein the instruments are coupled to the end of the wireline. Extending and withdrawing the wireline may be performed using a winch or similar spooling device known in the art. However, such conveyance relies on gravity to move the instruments into the wellbore, which can only be used on substantially vertical wellbores. Those wellbores deviating from vertical require additional force to move through the wellbore.
There are several types of wireline instrument conveyance known in the art for the foregoing conditions. One conveyance technique includes coupling the wireline instruments to the end of a coiled tubing having a wireline disposed therein. The wireline instruments are extended into and withdrawn from the wellbore by extending and retracting the coiled tubing, respectively. A subset of such coiled tubing techniques includes preliminary conveyance of the wireline configurable well logging instruments to a selected depth in the wellbore. See, for example, U.S. Pat. No. 5,433,276 issued to Martain et al. However, the use of coiled tubing with wireline instruments is costly and is inherently limited by the amount of pushing force capable with the coiled tubing. As a result, the use of coiled tubing is typically problematic in extended reach wells.
Another well logging instrument conveyance technique includes coupling wireline configurable well logging instruments to the end of a drill pipe or similar threadedly coupled pipe string. A wireline is coupled to the instruments using a “side entry sub” which provides a sealable passage from the exterior of the pipe string to the interior thereof. As the pipe string is extended into the wellbore, the wireline is extended by operating a conventional winch. An example of the foregoing is described in U.S. Pat. No. 6,092,416 issued to Halford et al. and assigned to the assignee of the present invention. However, this conveyance technique is frequently unreliable as the wireline is positioned in the annulus and subject to crushing, splicing or other damage. For example, the wireline may become pinched between the drill pipe and the casing or wellbore. Another drawback to using drill pipe to convey the well logging instruments using procedures known in the art is that the cable disposed outside the pipe disturbs the operation of the sealing equipment and makes it difficult to seal the drill pipe to maintain fluid pressure.
Additionally, the well logging instruments may be positioned at the end of a drill pipe without use of a wireline cable. In such circumstances, each well logging instrument is provided with a battery and memory to store the acquired data. As a result, the well logging instruments cannot communicate with the surface while downhole. Therefore, the data acquired cannot be analyzed at the surface until the wireline instruments return to the surface. Without any communication with the surface, surface operators cannot be certain the instruments are operating correctly, cannot control the instruments while downhole, and the data cannot be analyzed until after the wireline instruments are removed from the wellbore.
Recently, a type of drill pipe has been developed that includes a signal communication channel. See, for example, U.S. Pat. No. 6,641,434 issued to Boyle et al. and assigned to the assignee of the present invention. Such drill pipe, known as wired drill pipe, has in particular provided substantially increased signal telemetry speed for use with LWD instruments over conventional LWD signal telemetry, which typically is performed by mud pressure modulation or by very low frequency electromagnetic signal transmission.
In any of the foregoing examples, as well as other types of well logging instrumentation, a record is made of the measurements made by the various sensors in the instruments, typically with respect to time. A record is also made of the instrument depth with respect to time. In order to produce an accurate record of instruments sensor measurements with respect to the depth in the wellbore, it is necessary to have accurate time indexing of the recorded measurements. Certain types of instruments make measurements that are dependent on accurate time reference, for example, instruments that measure electromagnetic propagation and acoustic travel time properties. Accordingly, it is desirable to have accurate time references for the various measurements made by the instruments in the wellbore. Typically, time reference is provided by a master clock. The master clock is then synchronized to the various instrument clocks in the instrument string. What are needed are methods for precise synchronization of various instrument clocks to a master clock.